Obesity, which is an excess of body fat relative to lean body mass, is a chronic disease that is highly prevalent in modern society. It is associated not only with a social stigma, but also with decreased life span and numerous medical problems, including adverse psychological development, coronary artery disease, hypertension, stroke, diabetes, hyperlipidemia, and some cancers. (see, e.g., Nishina, et al., Metab. 43:554–558, 1994; Grundy and Barnett, Dis. Mon. 36:641–731, 1990; Rissanen, et al., British Medical Journal, 301:835–837, 1990).
Obesity remains a problem, and treatment has been limited. There is, therefore, a need to develop pharmaceuticals and treatment regimes effective in the alleviation of obesity.
A hallmark characteristic of obesity is an increase in white adipose tissue (WAT) mass that is largely due to accumulation of triacylglycerol. This increase in WAT mass is a key contributor to obesity-associated complications. Diacylglycerol O-acyltransferases (DGATs, EC 2.3.1.2) are membrane-bound enzymes that catalyze the terminal step of triacylglycerol biosynthesis. Two enzymes that display DGAT activity have been characterized: DGAT-1 (diacylglycerol O-acyltransferase type 1) (see, e.g., U.S. Pat. No. 6,100,077; Cases, et al., Proc. Nat. Acad. Sci. 95:13018–13023, 1998) and DGAT-2 (diacylglycerol O-acyltransferase type 2) (Cases, et al., J. Biol. Chem. 276:38870–38876, 2001). DGAT-1 and DGAT-2 do not exhibit significant protein sequence identity. Importantly, DGAT-1 null mice do not become obese when challenged with a high fat diet in contrast to wild-type littermates (Smith, et al., Nature Genetics 25:87–90, 2000). DGAT-1 null mice display reduced postprandial plasma glucose levels and exhibit increased energy expenditure, but have normal levels of serum triglycerides (Smith, et al., 2000), possibly due to the preserved DGAT-2 activity. Since DGAT-1 is expressed in the intestine and adipose tissue (Cases, et al., 1998), there are at least two possible mechanisms to explain the resistance of DGAT-1 null mice to diet-induced obesity. First, abolishing DGAT-1 activity in the intestine may block the reformation and export of triacylglycerol from intestinal cells into the circulation via chylomicron particles. Second, knocking out DGAT-1 activity in the adipocyte may decrease deposition of triacylglycerol in WAT. The phenotype of the DGAT-1 null mouse, along with the results of our studies with DGAT-1 inhibitors in diet-induced obese (DIO) mice, indicate that a DGAT-1 inhibitor has utility for the treatment of obesity and obesity-associated complications.